The present invention relates to the field of hydraulic dampers for actuator mechanisms and more particularly to a hydraulic damper with electro-rheological fluid which can vary the degree of dampening for the actuator mechanism.
The high rates of speed and reduced flight times of various missile systems often require that airframe responses to command controls be swift, accurate, and sustainable. Many missile systems use pneumatic, hydraulic, or electro-mechanical actuators for aerofin and thrust vector control.
Actuator mechanisms typically use a pressurized gas or fluid to drive moveable element such as a piston or set of pistons which in turn, rotate or move a control surface. These conventional actuator mechanisms have historically demonstrated several undesirable characteristics. Due to the compressibility of the gas or fluid medium the control surface may tend to flutter. Also, a common anomaly is the lack of precision of the actuator mechanisms in that the control surface may not move to the exactly to the commanded position.
Electro-pneumatic actuation mechanisms powered by compressed gases posses the advantages of being fast acting, clean, light weight, and relatively inexpensive to install, operate and maintain. A major drawback of purely pneumatic actuation mechanisms is the fact that the output motions cannot be precisely controlled. This is due primarily due to the tendency of the gases to demonstrate a large magnitude of compression and subsequent expansion. Pneumatic actuators do not possess the stiffness, speed and positioning accuracy necessary for many missile and rocket applications.
Electro-hydraulic actuation mechanisms can be more precisely controlled due to the relative incompressibility of the hydraulic fluid as compared to pneumatic gases. Active hydraulic actuation mechanisms, however, require various pumps and pump motors which introduce unwanted noise, vibration in addition to reliability concerns. Hydraulic actuation mechanisms tend to be somewhat expensive to install and maintain. The circuitry for conducting hydraulic fluid to and from the pump and associated components often poses operational and logistical constraints when used in confined spaces such as missile airframes. The hydraulic actuation mechanisms also include various fittings which introduce potential leakage paths for the hydraulic fluids operating under high pressures.
Hybrid hydro-pneumatic drive mechanisms are also used. These hydro-pneumatic drive mechanisms typically seize the advantages of purely electro-pneumatic drive mechanisms as well as the advantages of purely electro-hydraulic drive mechanisms. Such hybrid mechanisms generally employ an active pneumatic mechanism with a passive or pumpless hydraulic mechanism and are connected in such a manner that the movement imparted to the drive member by the pneumatic drive mechanism also produces a flow of fluid within the hydraulic mechanism. The restriction of this fluid flow further acts to control the drive member.
Such hydraulic dampening occurs when the motion of a piston causes fluid to flow through a restriction or orifice. The force required to move the piston at a given rate is proportional to the degree to which the fluid is restricted. Common methods to vary the restriction of the fluid are with the use spool valves and solenoid valves. Spool valves generally have smooth, continuously variable restriction but cannot provide complete restriction due to the tolerances necessary to allow the spool to slide. Solenoid valves, on the other hand, usually provide a few discrete restriction positions which correspond to discrete plunger positions. Solenoid valves can also provide complete restriction in at least one flow direction.
A drawback of the use of these current passive damping techniques is that a tradeoff must be made between actuator speed, and actuator stiffness. The term stiffness refers to the ability of an actuator to respond to and maintain command positions despite external disturbances such as the presence of vibrations and wind turbulence. Damping techniques are often used to influence actuator stiffness. Actuator stiffness increases with dampening while actuator speed often decreases with damping.